This invention relates generally to aqueous copolymer dispersions for used in stone paint or in other architectural exterior or interior coatings which needs superior water whitening resistance.
Water whitening resistance is required in many architectural coatings. Normally, when an aqueous latex emulsion dries, the latex particles are coalesced to form a film. Other components in the coating, such as surfactant, initiator, buffer, coalescent and preservative, which may be ionically-charged and water-soluble, are trapped in the interstitial areas of the film. These components are prone to attract water due to their hydrophilic chemical nature which endows the film a driving force by which the water can easily migrate from the film surface into the interstitial areas and thus cause the water-whitening problem.
The water whitening resistance of latex compositions can be improved by removing hydrophilic components, introducing special monomers into the polymer latex, or crosslinking the polymer latex. But these methods are still not ideal in process simplicity, cost effectiveness and performance balance.
Polyamine is used as an additive in acrylic copolymer dispersions for aqueous coating compositions. JP7026194A provides a water-borne dispersion for coating with improved adhesion to the substrate, hardness and stain-proof properties by formulating an acrylic copolymer (A) having −20° C. or higher glass transition temperature with a polyamine additive (B) at 0.05 to 40 (B)/100 (A) weight ratio. The monomers of (a) 0.1 to 40 wt % of an ethylenically unsaturated carboxylic acid; (b) 50 to 99.9 wt % of an alkyl(meth)acrylate; and (c) 0 to 49.9 wt % (where (a)+(b)+(c)=100 wt %) of other copolymerizable monomers are subjected to copolymerization to prepare copolymer (A). The water-borne dispersion shows good water resistance in performance evaluation. However, the water whitening resistance property of the dispersions are not mentioned in the reference. The inventors surprisingly found that, by adjusting the formulation copolymer composition and the content of the copolymer and the polyamine additive, the water whitening resistance performance of the dry coating film derived therefrom can be obviously improved.
Therefore, the problem addressed by this invention is to find copolymer dispersions for the application in coatings which show higher level of water whitening resistance of the dry coating film therefrom.